Abstract
Due to the low temperature and complex composition of the exhaust gas of the marine diesel engine, the working requirements of the selective catalytic reduction (SCR) catalyst cannot be met directly. Moreover, ammonium sulfate, ammonium nitrate, and other ammonium deposits are formed at low temperatures, which block the surface or the pore channels of the SCR catalyst, thereby resulting in its reduction or even its loss of activity. Considering the difficulty of the marine diesel engine bench test and the limitation of the catalyst sample test, a one-dimensional simulation model of the SCR system was built in this paper. In addition, the deactivation reaction process of the ammonium salt in the SCR system and its influencing factors were studied. Based on the gas phase and the surface reaction kinetics, the models of the urea decomposition, the surface denitrification, the nitrate deactivation, and the sulfate deactivation were both constructed and verified in terms of accuracy. Moreover, the formation/decomposition reaction pathway and the catalytic deactivation of ammonium nitrate and ammonium bisulfate, as well as the composition concentration and the exhaust gas temperature range were correspondingly clarified. The results showed that within a certain range, the increase of the NO2/NOx ratio was conducive to the fast SCR reaction and the NH4NO3 formation’s reaction. Increasing the exhaust gas temperature also raised the NO2/NOx ratio, which was beneficial to both the fast SCR reaction and the NH4NO3 decomposition reaction, respectively. Furthermore, the influence of the SO2 concentration on the denitrification efficiency decreased with the increase of the exhaust gas temperature because of increasing SCR reaction rate and reversibility of ammonia sulfate formation, and when the temperature of the exhaust gas was higher than 350 °C, the activity of the catalyst was almost unaffected by ammonia sulfate.
Highlights
Since marine diesel engines use low-quality fuel, their exhaust gases contain a lot of nitrogen oxides (NOx), sulfur oxide (SOx), particulate matter (PM), and other pollutants [1,2], which cause serious problems to both the marine environment and the atmosphere, respectively
Selective catalytic reduction (SCR) is the only technology recognized by the International Maritime Organization (IMO) that is used to reduce NOx emissions in all kinds of marine engines [5], which theoretically meets the IMO Tier III regulation and the other stricter emission standards
The main parameters of the catalyst used in the selective catalytic reduction (SCR) system are shown in the Table 1
Summary
Since marine diesel engines use low-quality fuel, their exhaust gases contain a lot of nitrogen oxides (NOx), sulfur oxide (SOx), particulate matter (PM), and other pollutants [1,2], which cause serious problems to both the marine environment and the atmosphere, respectively. Ammonium sulfate and ammonium nitrate can be generated when there is a large number of SOx and H2 O in the exhaust gas that blocks the surface and the pore channel of the catalyst [8,9], thereby reducing its activity. These ammonium salts mainly include ammonium sulfate ((NH4 )2SO4 ), ammonium bisulfate (NH4 HSO4 ), ammonium sulfite ((NH4 )2SO3 ), ammonium bisulfite (NH4 HSO3 ), ammonium nitrate (NH4 NO3 ), etc., which are relatively unstable and easy to decompose at high temperatures. It is necessary to build a simulation model of the marine diesel engine SCR system to study the influence of the ammonium salt passivation reaction on the SCR system
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